1. Field of the Invention
The present invention relates to a radar device for sending a radar wave in a predetermined reference direction, receiving as a reception wave the radar wave reflected by a target, and detecting the angle of the target relative to the reference direction. More particularly, the present invention relates to a radar device and a target angle detection method in which the above-mentioned angle is detected on the basis of a first phase difference of the reception wave received by a first receiving antenna pair that is spaced by a first gap, and a second phase difference of the reception wave received by a second receiving antenna pair that is spaced by a second gap smaller than the first gap.
2. Description of the Related Art
Known vehicle control systems for vehicle collision response include control systems in which the surroundings of a cruising vehicle are scanned by an on-board radar device, and the vehicle control system accelerates/decelerates the vehicle and/or activates safety devices when a collision with an obstacle is predicted. Examples of such on-board radar devices include, for instance, phase monopulse radar devices such as the one disclosed in Japanese Patent Application Laid-open No. 2001-51050, in which the angle of a target is detected on the basis of the phase of radar waves.
FIG. 1 and FIG. 2 are diagrams for explaining a method for detecting the angle of a target in a phase monopulse radar device. As illustrated in FIG. 1, a phase monopulse radar sends a radar wave W1 in a reference direction that corresponds to the front direction F of the radar device. In the phase monopulse radar device, a receiving antenna pair A1 constituted by two receiving antennas 11, 12 receives a radar wave W2, reflected by a target T, in the form of two reception waves W21, W22.
Herein, the gap d1 between the receiving antennas 11, 12 is very small compared to the distance to the target T, and hence the target T may be regarded as being at infinity. Such being the case, the incoming directions of the reception waves W21, W22 in the receiving antennas 11, 12, which give the angle θ of the target T (hereinafter, simply angle of the target) relative to the front F, which is the reference direction of the radar device, are identical. In consequence, the resulting difference Δd1 between the propagation distances of the reception waves W21, W22 is proportional to the gap d1 between the two receiving antennas. A phase difference arises as a result between the two reception waves. The angle θ of the target T can be obtained on the basis of equation 1 below, in which λ denotes the wavelength of the reception waves W21, W22 and Φ1 is the above-mentioned phase difference.θ=arcsin(λ·Φ1/(2Π·d1))  Equation 1
The relationship of Equation 1 is illustrated in FIG. 2A. In FIG. 2A, the axis of ordinate represents the phase difference of the reception waves and the axis of abscissa represents the angle of the target, with 0 degrees as the reference direction. Herein, an angle θ1 corresponding to the phase difference Φ1 is uniquely determined within a −10 degree to +10 degree range on the basis of the straight line L1, which corresponds to Equation 1.
The slope of the straight line L1 is proportional to the gap d1 between the receiving antennas 11, 12. Therefore, the slope of the straight line L1 becomes less steep, as illustrated by the broken line the figure, as the gap d1 decreases. In turn, the angle variation relative to variation in the phase difference Φ1 becomes greater; i.e. angle resolution decreases. Accordingly, the gap d1 between the receiving antennas 11, 12 is established in such a manner so as to obtain a slope of the straight line L1 that affords an adequate angle resolution. On the basis of the straight line L1 of FIG. 2A, however, the angle can only be determined within a −10 degree to +10 degree range. Therefore, the phase difference Φ1 may be replicated beyond ±Π in order to expand the detection range but without reducing angle resolution.
In such a case, as illustrated in FIG. 2B, an angle θ1 corresponding to the phase difference Φ1, an angle θ2 corresponding to a phase difference Φ1+2Π and an angle θ3 corresponding to a phase difference Φ1−2Π are determined on the basis of the phase difference Φ1, within a wider angle range than −10 degree to +10 degrees. However, the angle cannot now be uniquely specified on the basis of the phase difference Φ1.
Returning to FIG. 1, the phase monopulse radar device is provided with a receiving antenna pair A2 having a receiving antenna gap smaller than that of the receiving antenna pair A1. Specifically, the receiving antenna pair A2 is constituted by the receiving antenna 11 and a receiving antenna 13 spaced apart from the receiving antenna 11 by a gap d2.
The relationship between the angle of the target and a phase difference Φ2 of the reception wave at the receiving antenna pair A2 is given by a straight line L2, illustrated in FIG. 2C, having a shallower slope than the straight line L1. Angle θ1 corresponding to the phase difference Φ2 can be uniquely determined on the basis of the straight line L2 within a range from −20 degrees to +20 degrees. Although the angle can now be specified uniquely within a wider range than for the straight line L1, the angle resolution decreases in this case. The phase monopulse radar device, therefore, checks a plurality of angles θ1, θ2, θ3 determined with high angle resolution on the basis of the phase difference Φ1, against an angle θ1 determined with low angle resolution on the basis of the phase difference Φ2, and specifies the angle of the target by using the coinciding angle θ1 as the detection angle.
By virtue of its use for scanning obstacles around the vehicle, the above-described phase monopulse radar device should desirably have a wider target angle detection range. In the above-described method, though, the target angle detection range is restricted to an angle range that corresponds uniquely to the phase difference Φ2, namely a range from −20 degrees to +20 degrees, as illustrated in FIG. 2C. Expanding the angle detection range by making the slope of the straight line L2 yet shallower, however, is undesirable, since doing so results in lower angle resolution. Also, modifying to that end the gap between receiving antennas 11 and 13 involves substantial overhaul costs of the radar device when the latter is already fitted in the final product.